The present invention relates particularly to a press forming apparatus for sintered parts, which is suitable for producing spiral sintered parts.
A press forming apparatus for producing spiral parts such as an impeller used in a pump, a rotor for a compressor, and the like, is arranged, for example, such that a spiral mold cavity is formed in a die that is an outer mold, so as to extend vertically through the die, and a lower punch is fitted in the mold cavity so as to be movable vertically, while a vertically movable upper punch is arranged above the die. The arrangement is such that spiral compacts are formed by compression due to the upper and lower punches. In this compression, a withdrawal type is also utilized in which the lower punch is fixed, while the die is movable vertically.
Since the die is formed therein with the spiral mold cavity, the aforesaid conventional apparatus is left under a unsteady condition in which its center is insufficient in rigidity. For this reason, deformation occurs at compressive forming or molding and when a compact is withdrawn so that there is a fear that the press forming apparatus is broken and cracks are developed in the compact.
As a remedy for the above, an arrangement is known as is described in Japanese Patent Laid-Open No. SHO 61-232019, in which each of core sections located respectively between vortexes of the mold cavity is divided from the die, and the divided core is supported from the below by a plurality of rods. An arrangement is also known as is described in Japanese Patent Laid-Open No. HEI 2-61001, in which an auxiliary element is provided out of a forming range of a die having a spiral mold cavity to reinforce the die, and a lower punch is supported from the below and is provided with slots so as not to interfere with the reinforcing element.
However, since the former arrangement is of construction in which the divided core is supported through a plurality of rods by a core plate which is located below a lower punch receiving plate, the plurality of rods receive a pressure of compressive forming so that it is difficult to produce high density compacts.
The same is applicable also to the latter arrangement. Since at least two slots are formed in the lower punch, strength is low so that the arrangement is easy to be broken. Thus, it is difficult to produce high density compacts.